During the process of net shape molding an engineered part, liquid material such as molten plastic resin or magnesium alloy is injected into one or more mold cavities in a mold through gates provided in the mold. As the material is introduced into the mold cavities, the air which resides inside the cavities must escape through vents in the mold. Vents are typically located across a cavity from the gates. Venting also can occur through the parting lines formed between the two halves of the mold, typically called the cavity half and the core half of a mold. Vents are sized such that air will flow out of the vents but the moldable material will not follow it through the vents.
As the liquid to be molded into a part flows into a mold cavity, a thin skin is formed along the walls of the cavity as the material begins to solidify due to cooling or curing at the walls. The skin that is formed serves to prevent the material from flowing out through the vents and also contributes to the surface appearance and surface hardness of the parts being molded. Molds are then cooled (or heated or allowed to cure depending on the liquid material and the molding process being employed) to cause the material to solidify into the desired parts. As the material solidifies, some shrinkage typically occurs; the amount of shrinkage is dependent on the amount of material “packed” into the cavity. In parts requiring a higher degree of dimensional precision, the amount of shrinkage is accounted for in the design of the mold and in the selection of the moldable material being used to form the part.
The need for determining the condition of vents in molds is applicable to many different molding processes and materials. Among these are plastic injection molding, plastic blow-molding, liquid silicone molding, and magnesium alloy injection molding, to name a few. Plastic injection molding is a familiar technology, and the need for such checking of mold vent condition is easily understood within the context of plastic injection molding and thus will be discussed further in this context.
It is important that the total area of the venting in a mold cavity be neither too small nor too large. Too little venting can cause a number of problems in the plastic parts being molded. Examples of problems caused by not providing enough venting area in a mold cavity are: (a) warpage or other evidences of unwanted internal stresses in the parts after cooling; (b) short filling or voids (portions of part not formed due to inadequate material introduced into a cavity); (c) burn marks or fractured parts caused by combustion of trapped gases at very high pressure; and (d) poor surface finish caused by inadequate packing of the cavity with resin material.
The primary symptom of excessive venting in a cavity is the formation of flash on the part being molded in the cavity. Thus, there is an optimal amount of venting required in an injection mold cavity to produce a part of the highest quality.
In molds which contain multiple cavities, it is important that the venting be uniform cavity-to-cavity so that the multi-cavity mold obtain balanced filling of each cavity. All of the problems associated with improper (too little or too much) venting occur in multi-cavity molds with the added complexity that for all cavities of a mold to produce “good parts” during a molding cycle, uniform venting performance in each cavity must be achieved. Non-uniform shrinkage of the parts from different cavities or non-uniform weight of these parts are additional evidences of improper venting in multi-cavity molds.
Injection molding is generally used to produce a large number of parts from each mold through the repetitive cycling (filling, cooling, and part-ejection) of the injection molding process. During this large number of cycles, it is inevitable that material build-up occurs in a mold such that vents become blocked, parting lines may not align in the desired fashion, and/or cavity-to-cavity imbalances in venting can develop. Thus, a mold which is optimally set up to produce parts of the highest quality will over time produce parts of diminished quality.
With all of these possible vent-related causes of “bad parts” from the injection molding process, there is a need for a way to ensure that the desired amount of venting is provided in an injection mold cavity. One way to achieve excellent venting performance in an injection mold is to be able to measure reliably and repeatably to amount of venting in a mold cavity such that one can monitor how the venting is changing and take corrective action when the amount of venting moves out of a desired range. One method of attempting to achieve such a measurement is through application of pressurized air to a cavity and subsequent measurement of the time it takes for the air to leak from the cavity. Such approach is mentioned in the December 2003 issue (volume 6, number 12) of “Moldmaking Technology” magazine, page 17, in an article by the present inventor. Such pressure-based leak-down approach is unsuccessful in that it lacks adequate precision and sufficient repeatability for useful cavity-venting assessment.